1. Field of the Invention
This invention relates to information handling and control systems, manually operable electrical switches for use in such systems, and methods of testing such switches in such systems.
In particular this invention relates to an information handling and control system in which a manually operable electrical switch is provided in a low current signalling link to an intelligent unit which recognizes a binary input signal from the switch and in response provides an output signal in a low current signalling link to power switching means to control a high current to a load, and in which the switch includes two terminals and a conductive member which is movable such that in normal operation the resistance between two terminals with the movable member in a respective first or second position is recognized in the intelligent unit as a respective first or second condition of the binary input signal from the switch.
2. Description of the Related Art
An example of such a system in the form of an automotive vehicle electrical system is known from the article "Multiplexing for the automotive industry" by W. R. Betts in GEC REVIEW, Vol. 2, No. 1, 1986 at pages 32 to 36.
In such a known system if the manually operable switches are of conventional form, then in each switch a metal pin provides each of the two terminals and its respective contact and the movable member connects or disconnects the two contacts to provide short circuit resistance of open circuit resistance between the two terminals which is recognized as the respective first or second condition of the binary input signal from the switch.
A problem with conventional switch contacts, normally of copper, is the build up of oxide corrosion over a period of time. If the switch contacts are directly connected to the load which they control and carry the same high current, then making and breaking this high current helps to prevent oxidation build up. However, this protection is not available in systems of the type to which this invention relates in which the switch indirectly controls its associated load and is included in a low current signalling link. Precious metal contacts, normally gold plating, can be used to overcome the oxidation problem but this is expensive and can be unacceptably so in extremely cost sensitive very large scale mass produced systems such as automotive vehicle electrical systems. The low current signalling links in systems of the type to which this invention relates may operate at low voltage, for example 5 volts or 12 volts. These low voltages will not provide any significant protection against switch contact oxidation. The system described in the above-mentioned GEC REVIEW article uses a higher voltage of 30 volts on the low current signalling links and it is mentioned in the article that this use of 30 volts should reduce the need for precious metal contacts. However, it is considered that there may still be a long term oxidation problem.
An object of the invention is substantially to overcome the above-discussed problem of switch contact oxidation, and the invention is based on the realization that in systems of the type to which this invention relates it is not necessary that when the movable member connects the two switch contacts the condition of the binary input signal from the switch which is to be recognized must correspond to short circuit resistance between the two terminals.
According to the invention there is provided an information handling and control system as described in the second paragraph of this specification, characterized in that the switch includes two conductive plastics members each of which provides one of the two terminals and its respective contact, the movable member being arranged to connect the two contacts in said first position such that said first condition of the binary input signal from the switch is distinct from a short circuit condition in the signalling link to the intelligent unit.
An automotive vehicle electrical system may include an information handling and control system as described in the previous paragraph.
According to the invention there is also provided a switch for use in a system as described in either of the two previous paragraphs, the switch including said two conductive plastics members and said movable conductive member.
In a system according to the invention the switch may include a resistive member permanently connecting the two terminals, in said first position of the movable member said resistance between the two terminals being substantially that of the conductive plastics contacts and the movable member, and in said second position of the movable member said resistance between the two terminals being substantially that of the permanently connected resistive member such that said first and second conditions of the binary input signal from the switch are distinguishable from each other and from both said short circuit condition and an open circuit condition in the signalling link to the intelligent unit.
According to the invention there is also provided a switch for use in a system as just described, the switch including said two conductive plastics members, said movable conductive member and said permanently connected resistive member.
In the switch described in the preceding paragraph the permanently connected resistive member may be provided by a conductive plastics body portion of the switch which locates said two conductive plastics members providing the terminals and contacts.
In a switch according to the invention the movable member may be conductive plastics.
A switch according to the invention may be a toggle switch providing only one said binary input signal in response to which the intelligent unit provides an off-off said output signal for the high current to said load.
A switch according to the invention may be all plastics.
In a system according to the invention, said low current signalling link to an intelligent unit will preferably include said manually operable switch, a local unit associated with said switch and wiring from said switch to said local unit.
According to the invention there is also provided a method of testing said switch and said wiring in a system as just described without involving manual operation of the switch, in which method a fixed voltage supply is applied via the intelligent unit to said signalling link including said switch, and in which a voltage value at the intelligent unit responsive to the application of said fixed voltage supply and dependent on the electrical resistance of the signalling link is checked to determine whether the switch is in said respective first or second condition, or said wiring is short circuited, or said wiring is broken.
According to the invention there is also provided a method of testing said switch and said wiring before assembly with said local unit in a system as described in the penultimate paragraph and without involving manual operation of the switch, in which method a fixed voltage supply is applied from test equipment via the local unit to said switch, and in which a voltage value at the test equipment responsive to the application of said fixed voltage supply and dependent on the electrical resistance of said wiring and between the two terminals is checked to determine whether the switch is in said respective first or second condition, or said wiring is short circuited, or said wiring is broken.
The ability to test the satisfactory condition of the switch and its wiring without involving manual operation of the switch is clearly advantageous, and particularly such testability during the manufacture of very large scale mass produced systems, for example automotive vehicle electrical systems, can provide considerable cost saving.